Repellent activity of Nepeta grandiflora and Nepeta clarkei (Lamiaceae) against the cereal aphid, Sitobion avenae (Homoptera: Aphididae)

Abstract

Nepeta spp. (Lamiaceae) contain principally the nepetalactone isomers (4a S,7 S,7a R) and (4a S,7 S,7a S)-nepetalactone, which are also known to comprise aphid sex pheromones and aphid parasitoid attractants. Previous studies have shown that Nepeta cataria can be grown as a non-food crop for the commercial-scale production of (4a S,7 S,7a R)-nepetalactone in high purity, thus removing requirement for highly demanding stereocontrolled synthesis. Furthermore, previous literature reports have suggested that Nepeta spp., other than the widely studied N. cataria, can produce a diverse range of natural products other than nepetalactones, including high value sesquiterpenes. Thus, with the aim of identifying alternative Nepeta spp. for production of high value semiochemicals, the essential oils of two readily available species, Nepeta grandiflora and N. clarkei, were collected by steam distillation and analysed using high resolution gas chromatography (GC) and coupled GC–mass spectrometry (GC–MS). Analysis of the oils surprisingly revealed a very low nepetalactone content, but instead the presence of predominantly sesquiterpene compounds, and, for N. clarkei, the major component, geranyl acetate. Amongst the compounds identified in both oils was ( S)-germacrene d, a previously reported potent arthropod repellent. To determine the potential crop protection value of these essential oils, the repellent activity of the oils was assessed using a four-way olfactometer assay, with both oils showing strong repellent effects. These findings illustrate that members of the Nepeta genus possess a diverse range of high value secondary metabolites, and also highlight their potential to be developed as renewable feedstocks for the production of repellents for use economically important crop pests, as well as for the production of sex pheromones and natural enemy attractants.